As research unfolds about intermittent fasting (IF), it becomes more and more compelling as a viable dietary plan for weight loss, leaner body composition and perhaps even muscle gain. However, it must be said, some of the research does indeed steer a certain group of people away from it: namely, women.

As we’ve stated before, one of the main reasons we’re interested in intermittent fasting from a fat-loss perspective is because it seems to promote increased insulin sensitivity — the studies we’ve looked at have all shown this trait, and it makes sense even dating back to our research on diabetes prevention. However, it also appears that the insulin sensitivity increase is gender-specific: Yes, males will get the benefit of a more responsive pancreas after periods of fasting, including sleep and fasted exercise. But a 2005 study from Pennington Biomedical Research Center in Baton Rouge shows that while glucose tolerance is unchanged after fasting, women’s tolerance actually decreases — impaired glucose tolerance is a form of hyperglycemia that is a precursor to diabetes — and insulin response is virtually unchanged.

Another bit of research, a 2010 study from the Institute of Food Nutrition and Human Health at Massey University in New Zealand, found that women also got little to no benefit from fasted resistance training, with men clearly displaying an advantage in skeletal muscle gain. (Source)

The theories as to why these changes alter the sexes differently, of course, relate to the relationship between regular food intake and hormone production. On top of quite a few IF responses from women who say that it caused them to have irregular periods, fatigue and stress, still more published works back it up. Tests in lab rats illustrate a picture of intermittent fasting wreaking havoc with females’ reproductive, sexual and hormonal systems:

In female rats: Any degree of nutritional stress (fasting or mere caloric restriction) causes increased wakefulness (during the day, when they normally sleep), better cognition (for finding food), hyper alertness, and more energy. In short, female rats become better at finding and acquiring food when they fast, as if their bodies aren’t as well-equipped to deal with the stress of going without food. They also become less fertile, while the males actually become hornier and more fertile (probably to account for the females’ plummeting fertility). Ovary size drops (bad for fertility), adrenal gland size increases (which in rats indicates exposure to chronic stress), and menstrual cycles begin to dysregulate in proportion to the degree of caloric restriction. (Source)

In light of the research and information available pertaining to intermittent fasting and its effects on female mammals, we can’t endorse IF as a sensible means for dietary planning for women. While the research looks promising for men, and many have reported great results from intermittent fasting, it appears women need to have a more steady dietary pattern, not only to achieve fitness results, but to maintain a healthy hormonal balance.